C-, Sr-isotope stratigraphy of carbonate rocks from the Southern Espinha?o Ridge, Minas Gerais, southeastern Brazil

摘要

Neoproterozoic carbonate rocks comprise different stratigraphic units in the southern part of the Espinha?o Ridge, Minas Gerais, Brazil. C, O- and Sr-isotope analyses were carried out along four selected stratigraphic sections across these formations. These are: (i) the Rio Pardo Grande Formation in the upper portion of the Espinha?o Supergroup, sampled in section 3; (ii) Macaúbas Group laminated limestones (Tijucu?u Farm) and dolostone layers (Domingas Formation) have been respectively sampled along the so-called sections 1 and 2, and (iii) the lower stratigraphic units of the Bambuí Group, sampled in section 4. Laminated limestone samples from the Macaúbas Group have δ~(13)C values as high as 10.9‰ decreasing up section to -1.1‰ and ~(87)Sr/~(86)Sr values vary from 0.7072 to 0.7076, a range commonly observed in Cryogenian rocks. In section 2, dolomitic samples exhibit ~(87)Sr/~(86)Sr from 0.7076 to 0.7077 while in section 3, ~(87)Sr/~(86)Sr from 0.7074 to 0.7079. In section 4, ~(87)Sr/~(86)Sr values are around 0.7080. The values of ~(87)Sr/~(86)Sr observed in carbonate samples from the Macaúbas Group are similar to those observed in the Sr-isotope secular curve for the Neoproterozoic. Carbonate samples from the base of the Bambuí Group correlate with Ediacaran fingerprints, after the Marinoan (ca. 635 Ma) glaciation.